Use a 4- or 8-track recorder to record your instrumental track, depending upon how complex the track will be. A 4-track is sufficient for two guitars (or one guitar and piano), bass and drums, but it doesn't leave you much room for anything else. Using a drum machine will free up a track, since you can record the drum machine while you play another instrument, fitting both instruments on a single track. Likewise, if you have another musician available, you can combine instruments on a single track by playing together. Leave an open track for mixing down the tracks. If you can afford a slight increase in price, go with an 8-track machine.
Lay the drum track down first, which will help you establish the timing and rhythm of your instrumental track as you play the rest of the instrument parts.
Play the rhythm guitar part next, placing it on a track by itself.
Add your bass guitar to the mix on its own track next. If the bass guitar part is more prominent than the rhythm guitar track, consider playing the bass first.
Add a lead guitar part. Since this is an instrumental, the lead guitar part may be a melody that will take the place of a vocal, most noted in the style of Joe Satriani's melodic instrumentals, especially on his "Surfing with the Alien" album.
Mix all of your separate instrumental tracks down to a single track until your instrumental has the balance of tones you want. Your 8-track recorder will most likely have adjustment capabilities for sound levels and bass and treble properties. Some higher-end models will even allow you to add special effects to your musical track.